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The preliminary report of the investigation indicated that three of the first stage angular velocity sensors, responsible for yaw control, were installed in an incorrect orientation. As the error affected the redundant sensors as well as the primary ones, the rocket was left with no yaw control, which resulted in the failure. Telemetry data also indicated that a pad umbilical had detached prematurely, suggesting that the Proton may have launched several tenths of a second early, before the engines reached full thrust. More informatino here:
